After wafer fabrication, semiconductor chips or integrated circuit (IC) chips have to undergo several steps to be prepared for eventual utilization. After inspection and singulation, individual IC chips are picked up and attached to their carriers, such as lead frames. Then, the respective conductive pads on the IC chips are connected to inner leads of the lead frames through fine conductive wires, forming wire-bonded lead frame assemblies. Afterwards, the wire-bonded lead frame assembly will be encapsulated with a plastic molding compound, and the encapsulated lead frame assembly will further be trimmed, marked and tested before they are mounted to other devices for utilization.
The encapsulants commonly used to encapsulate electronic devices or IC chips are plastic compounds, including epoxy and silicone plastic compounds with fillers up to 80% by weight. The plastic molding compound serves four basic functions: (1) Physically supporting a lead system for electrically connecting the integrated circuit on the chips with an outside component system which will utilize the chip; (2) Protecting the IC chips from contamination, abuse, mechanical damage or breaking; (3) Chemically protecting the chips from environmental hazards, such as moisture, dust and gases that would interfere with the performance of the IC chips; and (4) Providing a thermal path for dissipating the heat generated when the IC chips are functioning. Compared to some other encapsulation technologies, plastic encapsulation has major advantages such as being light in weight, high in fabrication efficiency and low in manufacturing cost.
However, one disadvantage of plastic encapsulation is concerned with its non-hermetic sealing around the encapsulated electronic device or IC chip, which may result in water adsorption by the plastic compounds or moisture permeation through the compounds. This gives rise to a problem commonly associated with current molding compound technology. The problem can be exacerbated by a larger difference in the coefficient of thermal expansion (CTE) between the plastic compounds and carriers like lead frames. As the encapsulated assembly undergoes large and quick temperature changes, thermal stress within the encapsulated body may stimulate fine crazes on the interfaces, especially when the interface bonding is not strong enough. The fine crazes may develop into cracks under conditions of cycling thermal impact. The cracks provide routes for penetration by moisture. Consequently, it is easier for water to enter and accumulate in the encapsulated body. The adsorbed water not only speeds up chemical or metallurgical interaction for some IC chips, but can also lead to device failure in applications or even mounting processes.
For example, the adsorbed or entrained water will flash to steam when the encapsulated lead frame assembly is exposed to rapid heating during component assembly or device use. This will generate a rapid increase in volume locally. The rapid expansion may lead to delamination on three pairs of interfaces: the lead frame/plastic compound interface, the IC chip adhesive/chip attach paddle interface and the IC chip/plastic compound interface. As a consequence of the delamination, the normal IC functions or the connection of IC chips with an outside circuit are disrupted. Internal delamination can also disturb stress and strain distribution within the encapsulated body, which may further result in the fracture of wire-bonded IC chips or disruption of proper heat dissipation paths, and thereby impair the performance of the encapsulated electron devices. In more severe situations, the encapsulated body expands and even ruptures due to rapid hydraulic expansion, especially as the area ratio of carriers to IC chips becomes smaller in modern electronic packages. This phenomenon is normally referred to as “popcorn” in solder welding of encapsulated devices, and occurs more frequently in relation to surface mount assemblies.
Another problem related to plastic encapsulation comes from additives incorporated in encapsulants. The additives include coupling agents, flame-retardants, release agents and others. A commonly used flame-retardant additive in plastic molding compounds for semiconductor packaging are antimony compounds and brominated epoxy. The incorporation of flame retardants in encapsulation compounds is mandated by the fact that some encapsulated electronic devices have in the past generated such heat whereby the flash point of the molding compound was reached and fire ensued. In the event that encapsulant containing this flame-retardant system reaches its flash temperature, antimony compounds and brominated epoxy combine to form antimony tribromide, a dense and heavy flame-retardant gas. The gas prevents the flames from spreading. The adoption of the above-discussed flame retardant materials, as well as other known flame retardant materials, gives rise to another problem with current plastic encapsulation technologies. Some of these flame-retardant chemicals, such as brominated epoxy, when brought into contact with the encapsulated wire-bonded lead frame assembly, tend to degrade the reliability of wire joints. This degradation typically occurs as a result of the flame-retardant causing a degradation or even failure of the intermetallic joints between the bonding wire and at least one of the lead and/or the conductive pad on chips.
To avoid the delamination of plastic molding compounds from lead frames or substrates, many means have been proposed to improve the interfacial bonding. These means include utilizing mechanical interlocks and chemical bonding. Mechanical interlock involves impressions, such as holes, grooves and semi-spheres, being made mechanically on the lead frame, as described in U.S. Pat. No. 4,862,246 entitled “Semiconductor Device Lead Frame with Etched Through Holes” and U.S. Pat. No. 6,501,158 entitled “Structure and Method for Securing a Molding Compound to a Leadframe Paddle”. It was claimed that the impressions would increase the surface area of the lead frame and provide crevices for mechanical interlocking. Therefore, the adhesion of lead frames to plastic compounds was enhanced.
In another technique, black oxide has been successfully used for fabrication of printing circuit boards for some time. This technique was transferred to lead frame treatment, such as in U.S. Pat. No. 4,946,518 entitled “Method for Improving the Adhesion of a Plastic Encapsulant to Copper Containing Leadframes”. The main thrust of the technique is that the copper on the surface was oxidized in an active oxygen ambient and turned into black cupric oxide. The black cupric oxide has a needle structure on the scale of sub-microns. Thus, the surface area of the lead frame expanded significantly after treatment. Alternatively, by changing reaction conditions or partially converting cupric oxide into cuprous oxide through electric or chemical reduction, a brown oxide may be generated on the surface, as disclosed in U.S. Pat. No. 4,428,987 entitled “Process for Improving Copper-Epoxy Adhesion”. It is said that the brown oxide has a finer irregular structure than black oxide.
Coupling agents have also been used for adhesion for a long time. Normally, the coupling agents have two kinds of function groups that can react with substrates and adhesives respectively, so that they provide strong chemical bonding between the substrates and adhesives, such as disclosed in U.S. Pat. No. 6,369,452 entitled “Cap Attach Surface for Improved Adhesion”. However, copper-coupling agents may encounter hydrolysis under usual conditions of packaging.
It would be desirable to address the problem of weak bonding between interfaces without causing the degradation of wire joints of the lead frame assembly.